Figure 6.1 The bones and cartilages of the human skeleton. Epiglottis Larynx Thyroid cartilage Cartilage in external ear Cartilages in nose Trachea Cricoid cartilage Lung Articular Cartilage of a joint Cartilage in Intervertebral disc Figure 6.1 The bones and cartilages of the human skeleton. Costal cartilage Respiratory tube cartilages in neck and thorax Pubic symphysis Bones of skeleton Axial skeleton Meniscus (padlike cartilage in knee joint) Appendicular skeleton Cartilages Hyaline cartilages Elastic cartilages Fibrocartilages

(c) Flat bone (sternum) (a) Long bone (humerus) (b) Irregular bone (vertebra), right lateral view (d) Short bone (talus)

Articular cartilage Compact bone Proximal epiphysis Spongy bone Epiphyseal line Periosteum Endosteum Compact bone Medullary cavity (lined by endosteum) (b) Yellow bone marrow Diaphysis Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries Distal epiphysis (a) (c)

Figure 6.3a The structure of a long bone (humerus of arm). Articular cartilage Proximal epiphysis Spongy bone Epiphyseal line Figure 6.3a The structure of a long bone (humerus of arm). Periosteum Compact bone Medullary cavity (lined by endosteum) Diaphysis Distal epiphysis (a)

Articular cartilage Compact bone Spongy bone

Endosteum Yellow bone marrow Compact bone Periosteum Perforating (Sharpey’s) fibers Nutrient arteries

Figure 6.4 Comparison of different types of bone cells. (a) Osteogenic cell (b) Osteoblast (c) Osteocyte (d) Osteoclast Figure 6.4 Comparison of different types of bone cells. Stem cell Matrix-synthesizing cell responsible for bone growth Mature bone cell that maintains the bone matrix Bone-resorbing cell

Spongy bone (diploë) Figure 6.5 Flat bones consist of a layer of spongy bone sandwiched between two thin layers of compact bone. Compact bone Trabeculae

Figure 6.7 Microscopic anatomy of compact bone. Spongy bone Central (Haversian) canal Perforating (Volkmann’s) canal Figure 6.7 Microscopic anatomy of compact bone. Endosteum lining bony canals and covering trabeculae Osteon (Haversian system) Circumferential lamellae (a) Perforating (Sharpey’s) fibers Lamellae Periosteal blood vessel Periosteum Nerve Vein Artery Lamellae Central canal Canaliculi Lacuna (with osteocyte) Osteocyte in a lacuna Lacunae Interstitial lamellae (b) (c)

Figure 6.8 Intramembranous ossification. Mesenchymal cell Osteoblast Collagen fiber Osteoid Ossification center Osteocyte Newly calcified bone matrix Osteoid Osteoblast Figure 6.8 Intramembranous ossification. 1 Ossification centers appear in the fibrous connective tissue membrane. 2 Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies. • Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. • Osteoblasts begin to secrete osteoid, which is calcified within a few days. • Trapped osteoblasts become osteocytes. Mesenchyme condensing to form the periosteum Fibrous periosteum Osteoblast Plate of compact bone Trabeculae of woven bone Diploë (spongy bone) cavities contain red marrow Blood vessel 3 Woven bone and periosteum form. 4 Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. • Accumulating osteoid is laid down between embryonic blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae called woven bone. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. • Trabeculae just deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates. • Spongy bone (diploë), consisting of distinct trabeculae, per- sists internally and its vascular tissue becomes red marrow.

Figure 6.8 Intramembranous ossification (1 of 4). Mesenchymal cell Collagen fiber Figure 6.8 Intramembranous ossification (1 of 4). Ossification center Osteoid Osteoblast Ossification centers appear in the fibrous connective tissue membrane. • Selected centrally located mesenchymal cells cluster and differentiate into osteoblasts, forming an ossification center. 1

Figure 6.8 Intramembranous ossification (2 of 4). Osteoblast Osteoid Figure 6.8 Intramembranous ossification (2 of 4). Osteocyte Newly calcified bone matrix Bone matrix (osteoid) is secreted within the fibrous membrane and calcifies. • Osteoblasts begin to secrete osteoid, which is calcified within a few days. • Trapped osteoblasts become osteocytes. 2

Figure 6.8 Intramembranous ossification (3 of 4). Mesenchyme condensing to form the periosteum Trabeculae of woven bone Figure 6.8 Intramembranous ossification (3 of 4). Blood vessel Woven bone and periosteum form. • Accumulating osteoid is laid down between embryonic blood vessels in a random manner. The result is a network (instead of lamellae) of trabeculae called woven bone. • Vascularized mesenchyme condenses on the external face of the woven bone and becomes the periosteum. 3

Figure 6.8 Intramembranous ossification (4 of 4). Fibrous periosteum Osteoblast Plate of compact bone Figure 6.8 Intramembranous ossification (4 of 4). Diploë (spongy bone) cavities contain red marrow Lamellar bone replaces woven bone, just deep to the periosteum. Red marrow appears. • Trabeculae just deep to the periosteum thicken, and are later replaced with mature lamellar bone, forming compact bone plates. • Spongy bone (diploë), consisting of distinct trabeculae, per- sists internally and its vascular tissue becomes red marrow. 4

Figure 6.9 Endochondral ossification in a long bone. Week 9 Month 3 Birth Childhood to adolescence Articular cartilage Secondary ossification center Spongy bone Epiphyseal blood vessel Figure 6.9 Endochondral ossification in a long bone. Area of deteriorating cartilage matrix Epiphyseal plate cartilage Hyaline cartilage Medullary cavity Spongy bone formation Bone collar Blood vessel of periosteal bud Primary ossification center Bone collar forms around hyaline cartilage model. 1 Cartilage in the center of the diaphysis calcifies and then develops cavities. 2 The periosteal bud invades the internal cavities and spongy bone begins to form. 3 The diaphysis elongates and a medullary cavity forms as ossification continues. Secondary ossification centers appear in the epiphyses in preparation for stage 5. 4 The epiphyses ossify. When completed, hyaline cartilage remains only in the epiphyseal plates and articular cartilages. 5

Resting zone Proliferation zone Cartilage cells undergo mitosis. 1 Figure 6.10 Growth in length of a long bone occurs at the epiphyseal plate. Hypertrophic zone Older cartilage cells enlarge. 2 Calcification zone Matrix becomes calcified; cartilage cells die; matrix begins deteriorating. 3 Calcified cartilage spicule Osteoblast depositing bone matrix Ossification zone New bone formation is occurring. Osseous tissue (bone) covering cartilage spicules 4

Figure 6.11 Long bone growth and remodeling during youth. Bone remodeling Articular cartilage Cartilage grows here. Figure 6.11 Long bone growth and remodeling during youth. Epiphyseal plate Cartilage is replaced by bone here. Bone is resorbed here. Cartilage grows here. Bone is added by appositional growth here. Cartilage is replaced by bone here. Bone is resorbed here.

GIGANTISM

Figure 6.17 Fetal primary ossification centers at 12 weeks. Parietal bone Occipital bone Frontal bone of skull Mandible Figure 6.17 Fetal primary ossification centers at 12 weeks. Clavicle Scapula Radius Ulna Ribs Humerus Vertebra Ilium Tibia Femur

Hematoma 1 A hematoma forms.

2 External callus Internal New callus blood (fibrous vessels tissue and cartilage) New blood vessels Spongy bone trabecula 2 Fibrocartilaginous callus forms.

Bony callus of spongy bone 3 Bony callus forms.

Healed fracture 4 Bone remodeling occurs.

STRESS FRACTURE

Figure 6.12 Parathyroid hormone (PTH) control of blood calcium levels. Calcium homeostasis of blood: 9–11 mg/100 ml BALANCE BALANCE Stimulus Falling blood Ca2+ levels Figure 6.12 Parathyroid hormone (PTH) control of blood calcium levels. Thyroid gland Osteoclasts degrade bone matrix and release Ca2+ into blood. Parathyroid glands Parathyroid glands release parathyroid hormone (PTH). PTH

TRACTION